Negative indices
To index this plane it is useful to use a different corner of the unit cell as the point of reference.
Go along the positive x and z axes to find those intercepts.
Then go backwards along the negative y-axis, to get a negative intercept.
Instead of using a minus sign, we use an 'overbar' to denote a negative index. This gives a Miller Index of:
(112)
This reads as "one, bar one, two".